1. Field of the Invention
The present invention relates to a regenerable electric capacitor that has metal layers that have thicknesses and alloy compositions which are different dependent transverse to a direction of run of wound-on layers of plastic foils.
2. Description of the Prior Art
The possibilities of use of power capacitors are determined essentially by their thermal characteristics. In self-healing, or regenerable, capacitors, the greatest part of the losses arises in the metal layers of the coatings.
In order to minimize the losses, the metal layers can be made thicker and the surface resistances made smaller. However, for reasons of regeneration reliability, narrow limits are placed on this procedure since, beginning at a certain thickness of the metal layers a correct regenerability is no longer ensured. It is thus generally known that the thickness of at least one metal layer must be kept thin.
Improvements have resulted by means of a capacitor of the type named above known from DE 28 26 481 C2. in the capacitor depicted there, the metal layers, consisting of an alloy of 15-80 atomic % aluminum with copper, include a variable thickness perpendicular to the longitudinal direction of the foils) i.e., transverse to the direction of run of the capacitor foil) so that the cross-sectional profile of the metal layers is essentially wedge-shaped with a constantly increasing thickness. However, with respect to the regeneration characteristic, the most unfavorable conditions hereby prevail in the center so that the minimal allowable surface resistance is determined at this point.
In addition, from EP 0 088 137 A1, a self-healing electric capacitor is known in which the metal layers consist of an Al/Zn alloy whereby the Al portion constantly decreases from 80% at the side of the metal layer facing the foil to less than 20% at the side of the metal layer facing away from the foil.
If the manufacturing of wound capacitors is regarded from an economical point of view, it is to be noted that the manufacture of larger winding units is generally more cost-effective. For this reason, it is desirable to manufacture the largest possible winding units with large overlappings of the oppositely poled metal layers. However, for the thermal economy of the capacitor there results the disadvantageous effect that the series losses increase proportionally to the size of the covering so that an optimization problem between the economical winding manufacture and the thermal requirements occurs.
An object of the present invention is, therefore, to develop a capacitor that, in comparison with the prior art, exhibits an at least equally good, but if possible a better, regeneration characteristic and an equally good, or better, life span. The operation of such capacitor should be such that minimal coating losses arise which thus enable a higher exploitation of the dielectric material as well as cost reductions.